This invention relates to a magnetic bubble domain structure and more particularly to a magnetic bubble domain structure having an enhanced storage density area.
In recent years significant interest has developed in a class of magnetic devices generally referred to as magnetic bubble domain devices or "magnetic bubbles". These devices are described, for example, in IEEE Transactions on Magnetics, Vol. MAJ-5, No. 3 (1969), pp. 544-553, "Application of Orthoferrites to Domain-Wall Devices". These magnetic domain devices are generally planar in configuration and are constructed of materials which have magnetically easy directions which are essentially perpendicular to the plane of the structure. Magnetic properties such as magnetization and anisotropy, coercivity, and mobility, are such that the device may be maintained magnetically saturated with magnetization in a direction out of the plane, in that small localized single domain regions of magnetic polarization aligned opposite to the general polarization may be supported. Such localized regions which are generally cylindrical in configuration represent binary memory bits. Interest in these devices in large part is based on the high bit density that can be obtained and the ability of the cylindrical magnetic domain to be independent of the boundary of the magnetic material in the plane in which it is formed and to be capable of moving anywhere in the plane of the magnetic material to effect various data processing operations. Since the magnetic bubbles can be propagated, erased, replicated and manipulated in performing a data processing operation and their presence and absence detected, these bubbles may be utilized to perform the primary functions vital to memory operation.
Many structural organizations of operable magnetic domains have been disclosed in the literature. One of the most popular is the major-minor loop memory organization disclosed in U.S. Pat. No. 3,618,054. The major-minor loop memory organization as well as its implementation and operation is well known in the art. The major-minor loop organization includes a closed major loop which typically is established by an arrangement of propagation permalloy elements on, for example, a rare earth garnet film. The magnetic domains are propagated around the loop by in-plane rotating magnetic field action. The major loop is generally elongated to permit a number of minor loops to be aligned along side it. Two-way transfer gates permit the transfer of magnetic domains from each of the minor loops to the major loop and from the major loop to the minor loops. Further access to the major loop is achieved by a detecting read connection thereto and by a separate write connection.
An advance in the sophistication of chip architectures such as the major-minor loop architecture described above includes the "block-replicate" chip architecture which is generally described in an article entitled "64K Fast Access Chip Design" by J. E. Ypma, I. S. Gergis and J. L. Archer appearing in AIP Conference Proceedings No. 29 Magnetism and Magnetic Material, pp. 51-53 (1975). This type of magnetic bubble domain structure enables faster access to data, by permitting data readout at a rate equal to the field access rate. Structurally there are three sections to the block replicate chip architecture, the first being an input path, the second a storage section and the third an output path. The input path is associated with generating units to generate magnetic bubble domains to be stored in the storage section which may include a plurality of loop structures. The output path is associated with detector units and replicate gates for replicating the magnetic bubbles in the storage region by splitting the bubble into two sections and retaining one section in its virtual position in the storage section while transferring the second bubble into the output path to be moved to a detector unit. The storage section may be separated into an odd and even plurality of loops so as to permit alternating read out of data from the odd and even loops by the detector at a rate equal to the field rate of the device.
In both of the architecture structures described above, propagation elements made of permalloy material make up the propagation paths and storage sections. There are two basic types of circuit propagation element structures commonly employed in a magnetic bubble domain device. The first such structure comprises alternating T and bar-shaped elements, i.e. the so-called T-bar configuration, and the second structure comprises chevron elements. In the T-bar form of propagation elements, several characteristics occur that are limiting in nature. The first of these characteristics is the small gap between successive T and bar elements which inhibits further reduction in the size of these elements based on the present state of the optical lithography art. A second characteristic that is limiting for T-bar circuits is shape anisotropy causing the bars of permalloy to be amenable to strong bubble-bubble interaction. In forming a loop structure using T-bar permalloy elements, the loops are interconnected with the bar element thus enhancing the possibility of further bubble-bubble interaction.
The chevron-shaped elements comprising the second common type of circuit propagation element structures are gap tolerant elements. Where the chevron type elements are employed, the gaps between successive chevron elements may be designed to a greater value than possible with the T-bar structure. The chevron element is a single element structure which does not require interconnection between loops. These characteristics minimize any bubble-bubble interaction.
A magnetic drive field within the plane of the layer of magnetic material is rotated which causes the individual propagation elements, T-bar or chevron type, in the patterned bubble propagation paths to be sequentially polarized in a cyclical sequence causing the individual bubble domains to be propagated in a step wise movement along the path as defined by the magnetizable propagation elements. A bias magnetic field is also present which sustains the character of the bubble domain. An extreme increase in the bias field will cause the bubble domains to collapse and thus the reliability as a memory device depends upon control of the bias field. A bias field that is too low will cause a stripout phenomenon in which the bubble will elongate and thus become unusable. The point above the stripout field amplitude is that point at which a bubble will come into existence. A measurement of the bias field as a function of the drive field can be defined as a propagation margin and monitored at a variety of areas on the magnetic layer. The propagation margin is in actuality the difference between the collapse field and the stripout field as a function of the drive field. A magnetic domain structure is designed such that it will utilize the greatest propagation margin available considering the other functional limitations of the device, including replicating, transferring or detecting magnetic bubbles. The propagation margin when measured along the straight line of a loop structure found in the bubble storage section of a major-minor loop architecture or block replicate architecture has a value greater than the margin of the entire system. The optimum device will have a propagation margin such that the operating magnetic field will have a plus or minus variance between collapse and stripout large enough such that continuous operation in a reliable manner can be performed.
One of the main reasons for the interest in magnetic bubble domain structures is their potential for greater storage density. Neighboring bubbles are usually separated by 4.5-5.5 times the nominal bubble diameter, the average bubble size at the operating range of the bias field, in order to minimize the effect of bubble-bubble interaction which tends to reduce the propagation margins. Designing circuits with large bubble separating distances known as the circuit period is especially important with T-bar circuits mainly because margins are quite poor in the first place and also because adjacent tracks are normally connected as described above. In the gap tolerant structures, e.g. asymmetrical chevron type propagation elements, the margins are typically about 20% of bubble collapse for circuits in which the period is approximately 5 bubble diameters.
The goal in any memory device is to enhance the storage data capability of the structure. In the magnetic bubble memory device, one possibility of enhancing the storage capability would be to reduce the bubble size. However, it has been found that reduction of the bubble size enhances step coverage problems and is further limited by the state of the art of bubble materials, device processing, optical lithography, etc.